1. Statement of the Technical Field
The invention concerns communications systems. More particularly, the invention concerns a communications system receiver configured to (a) improve the effective signal-to-noise ratio (SNR) of a spread spectrum signal and (b) recover input data from the spread spectrum signal.
2. Description of the Related Art
There are many types of communications systems known in the art, such as multiple access communications systems, low probability of intercept/low probability of detection (LPI/LPD) communications systems and spread spectrum communications systems. Many of these systems depend on square wave symbol and/or spreading sequences. Other systems induce exploitable correlations via square-wave pulse shaping. Non-square wave spreading sequences (including chaotic spreading sequences) have also been employed but require significantly more computational power to synchronize. Communication signals employing non-square wave spreading sequences are typically more secure and robust to interferers. As described herein, a chaotic spreading sequence consists of a sequence of numbers having values that appear to have unpredictable transitions characteristics following that of a mathematically chaotic evolution and near ideal statistical properties, yet follow a well-defined deterministic evolution.
Such communications systems can also be configured to operate such that a communications signal has a spread power level below the noise floor of the environment as measured at the communication system receiver. The term “noise floor” as used herein refers to the level of noise which exists in a signal, measured at the input of a communication system receiver.
A multiple access communications system can handle a greater number of users as compared to conventional communications systems, whereby users typically simultaneously re-use a shared communications bandwidth. Each additional signal (optimally orthogonal) may be treated as additive noise. Also, an LPI/LPD communications system can generate signals having less detectable features (or lower measurable power levels) as compared to conventional communications systems since the ambient noise power masks the signal. The signals generated by such communications systems can operate at relatively low signal-to-noise ratios (SNRs), which forces reconstruction and data decision to rely on stochastic signal processing. Any method or device that improves this low SNR will improve the data estimation capability of the communications system, permitting some combination of increased throughput, reduced transmit power, or lower probability of detection/exploitation.